1. Technical Field
The present invention relates generally to a method for maintaining an orbiting satellite's pointing direction fixed, and more particularly, to a method for compensating for magnetic disturbance torques inflicted on a satellite in an earth orbit.
2. Discussion
A geosynchronous earth orbit, as is known in the art, is the orbit about the earth in which a satellite or spacecraft will remain fixed above a specific location on the earth. This orbit is at a distance of approximately 22,400 miles above the earth. In this orbit, a beam, such as a communications beam, from the satellite can be maintained over a desirable area on the earth, such as a particular country, thus establishing an area which will receive the beam. To remain in a geosynchronous orbit it is necessary that the satellite be in an orbit substantially within the equatorial plane of the earth at the desirable distance, and that the satellite's attitude be oriented perpendicular to this plane. Any deviation or disturbance which causes the satellite to direct its antenna away from a boresight location on the earth tends to effect the coverage area of the beam, and thus, produces undesirable results. Many different forces are in effect on the satellite which tend to alter the satellite's antenna pointing direction.
As a first order method for countering the effects of the different forces acting on the satellite, it is known to stabilize the satellite's attitude by providing an angular bias momentum which resists changes in the satellite's orientation due to external forces transverse to the bias momentum axis. Satellites using this technique are generally referred to as "momentum bias" satellites. Angular momentum bias is usually provided by a number of momentum or reaction wheels which spin at least part of the satellite. The bias axis set by the spin of the momentum wheels is generally perpendicular to the direction of the orbit of the satellite. Although the bias momentum resists changes in the satellite's orientation in directions transverse to the bias momentum axis, it is still necessary to provide control for correcting variations in the satellite's orientation along the bias axis. Different methods of controlling the satellite's attitude, such as feedback loops, are known in the art.
For most bias momentum satellites, the satellite payload, i.e., the part of the satellite carrying at least the antenna, is oriented differently than the momentum wheel. It is therefore necessary to provide means for correcting the orientation of the payload with respect to the orientation of the momentum attitude. Typically, the satellite's payload is defined in three axes referred to as the yaw, roll and pitch axes. If the satellite is in a geosynchronous orbit, the yaw axis is generally directed from the satellite to the center of the earth, the pitch axis is generally directed normal to the plane of the orbit of the satellite and the roll axis is generally perpendicular to the yaw and pitch axes, in a direction of travel of the satellite as is well known in the art.
As discussed above, different forces or disturbances act on an orbiting satellite causing the satellite to direct its pointing away from the desirable boresight location. One of those forces is caused by the earth's magnetic field. In the absence of magnetic storms, the earth's magnetic field at a geosynchronous altitude is about 110 nano-teslas oriented along the north pole of the earth. The earth's magnetic field can interact with electrical current loops on the satellite to produce a deviating torque to the satellite's attitude. Certain satellite wire harness designs may result in large current loops on the satellite, especially during periods of battery discharge such as would occur during an eclipse of the satellite's photodetectors when the satellite is obstructed from the sun by the earth. For example, a pointing error of approximately 0.2.degree. in the yaw direction would occur due to a 60 .mu.Nm disturbance torque in the yaw direction applied to the satellite during a one hour eclipse period. Such a torque would arise from a current loop in the roll direction on the satellite of approximately 200 A-turn-m.sup.2. Consequently, a significant pointing error will occur in the yaw direction altering the satellite's boresight location.
What is needed then is a method of compensating for the deviations in a satellite's attitude caused by the forces of the earth's magnetic field on the wiring in the satellite. It is therefore one objective of the present invention to provide such a method.